In recent years, electric double layer capacitors capable of charging and discharging with a large current have been attracting attention as electric power storage devices that hold promise for applications requiring frequent charge/discharge cycles, for example, as auxiliary power sources for electric vehicles, solar cells, wind power generation, etc. There is therefore a need for an electric double layer capacitor that has high energy density, is capable of fast charging and discharging, and has excellent durability.
An electric double layer capacitor comprises a pair of polarizable electrode layers as an anode and cathode disposed opposite each other with a separator interposed therebetween. Each polarizable electrode layer is impregnated with an aqueous or non-aqueous electrolytic solution, and is united with a current collector. With the aqueous type electrolytic solution, capacitance density can be increased to reduce resistance, but the operating voltage must be set lower than the voltage at which the electrolysis of water takes place; therefore, from the standpoint of increasing the energy density, the non-aqueous type is preferred for use.
In the past, electric double layer capacitors have been used primarily for such applications as memory backup and energy storage, but in recent years, electric double layer capacitors have been gaining attention for applications that require large current discharge, such as uninterruptible power supplies, and applications that require discharging under low temperature conditions, such as engine starters for hybrid electric vehicles, etc. For memory backup and energy storage applications, the capacitance density of the capacitor is an important factor, and to increase the capacitance density, activated carbon containing predominantly micropores of pore diameter smaller than 20 angstroms and thus providing a large specific surface area is used for polarizable electrode layers. However, if such polarizable electrode layers are used in applications that require a current of 10 mA/cm2 or larger, problems such as the decrease of capacitance density, increase of energy loss, increase of heat generation, etc. become pronounced, because the mobility of electrolyte ions within the pores is limited. There is also the problem that, after long use, residues of decomposed materials may clog the pores, which can significantly degrade the durability.
In memory backup and energy storage applications, the capacitor is discharged at a relatively small current over time, but if polarizable electrode layers formed from predominantly microporous activated carbon, such as described above, are used in applications where the capacitor is discharged at a large current in a short time, the discharge efficiency decreases. This is because the time constant RC given by the product of internal resistance R and capacitance C is large, reducing the discharge efficiency which is given by the equation: (1−2RC) [time constant)/t[discharge time]. A method is known that aims to improve the performance by reducing the ratio of the specific surface area of the micropores in the activated carbon used for the polarizable electrode layers or by increasing the specific surface area or pore volume of mesopores so that the capacitor can be used in large current, quick discharge applications (Japanese Unexamined Patent Publications Nos. 2000-340470, H08-119614, 2001-316103, and 2001-89119). According to this method, the mobility of electrolyte ions within the pores of the activated carbon increases, serving to suppress the performance drop and the discharge efficiency drop that occur when the capacitor is used at a current of 10 mA/cm2 or larger. However, none of Japanese Unexamined Patent Publications Nos. 2000-340470, H08-119614, 2001-316103, and 2001-89119 pay attention to the mobility of electrolyte ions in spaces other than the pores of the activated carbon (that is, the spaces in the interstices between the particles of the activated carbon, carbon black, etc. in the polarizable electrode layers), and it cannot be said that the method is effective enough in suppressing the performance drop. Especially, in the case of a large current discharge at 100 mA/cm2 or larger, the mobility of electrolyte ions is insufficient, and problems such as the decrease of capacitance density, increase of energy loss, increase of heat generation, etc. become pronounced. On the other hand, in the case of a discharge at low temperatures of −20° C. or lower, the resistance increases and, as a result, the capacitance density decreases, thus limiting applications in a low-temperature environment.
On the other hand, with such predominantly mesoporous activated carbon, since its bulk density is small compared with predominantly microporous activated carbon, the capacitance density tends to decrease. Accordingly, in the case of predominantly mesoporous activated carbon, there is employed a method that increases the electrode density by compacting the activated carbon particles as closely as possible in order to compensate for the decrease in capacitance density. One common method employed to increase the density is to broaden the particle size distribution of the activated carbon and to fill the interstices of the large size activated carbon particles with smaller size activated carbon particles (Japanese Unexamined Patent Publication Nos. 2005-317642, 2003-347172, and 2000-344507).
It is also known to provide a method that mixes a thermoplastic resin, soluble polymer, etc. when molding the electrodes, and thereafter forms microscopic voids by heat treatment or cleaning in the interstices between the particles in order to ensure the mobility of electrolyte ions in spaces other than the pores of the activated carbon (that is, the spaces in the interstices between the particles of the activated carbon, carbon black, etc. in the polarizable electrode layers) (Japanese Unexamined Patent Publication Nos. H10-208985, H07-99141, and 2000-113876). Such methods, however, have had the problem that it is difficult to control the voids and the capacitance density decreases excessively. Furthermore, when performing heat treatment, there have been such problems as the pores of the activated carbon being shrunk under heat and an inability to use a polymer binder because of its heat resistance problem, and when performing cleaning by dissolving in a solvent, there has been the problem that the durability decreases because the solvent is adsorbed on the activated carbon; because of these and other problems, the above method has not been practicable.
There is also known a method that aims to enhance the performance by increasing the porosity of the electrodes as a whole (Japanese Unexamined Patent Publication No. H01-227417). In the method disclosed in Japanese Unexamined Patent Publication No. H01-227417, the mixing ratio of the activated carbon is adjusted, and the electrodes are produced by drawing, but with this method, since the porosity is increased by increasing the interstices between the particles, the capacitance density significantly drops. Furthermore, in Japanese Unexamined Patent Publication No. H01-227417, there is no mention of the capacitance density, and attention is focused only on the amount of volume change after a high-temperature load test.
Various kinds of materials are used as materials for activated carbon for capacitors; among others, natural (plant-derived) materials (such as coconut shells, wood, etc.) are used, because such materials are inexpensive and already contain numerous fine pores inherent in the materials before activation, and it is therefore easy to reduce the resistance. However, the fabrication of the polarizable electrode layers using such natural activated carbon has involved the problem that the material is difficult to mold because of such problems as the crushing of particles during molding and the sliding characteristics of the particles. That is, cracks can easily occur in the polarizable electrode layers, the reduction in film thickness by rolling is small, and so forth. As a result, to improve the moldability, an increased amount of solvent (molding agent) has had to be added when kneading the material. The solvent is removed by drying after molding the polarizable electrode layers, but it is difficult to completely remove the solvent, and if the solvent remains in the pores of the activated carbon, the durability of the capacitor decreases.
Further, there are cases where a low-viscosity nitrile-based organic solvent, such as an acetonitrile solvent, is used in order to improve the large current and low temperature characteristics, but since nitrile-based organic solvents generate toxic cyanide gas when burning, using such solvents is not desirable from the standpoint of safety.